I am divorced, we split room and board for my son's school. I also took 529 funds to help us both but now I am paying income tax. Do I enter 1/2 of the total?

If all of the 529 plan distributions were used for qualifying expenses then you don't need to enter anything from the 1099Q  in the program or on the return at all.  

Room and board (R&B) are qualified expenses for a 529 plan distribution.  R&B are not qualified expenses for tax free scholarship and/or an education credit or deduction.  

So, unless the other parent also took money out of a different 529 plan, you can claim all the R&B expenses. You don't have to split/share them. Also,  as Critter-3 said, if the entire distribution was used for R&B, you do not need to enter the 1099-Q, at all.  When the box 1 amount on form 1099-Q is fully covered by expenses, TurboTax will enter nothing about the 1099-Q on the actual tax forms. 

However, if the 529 distribution exceeded R&B and was used to pay other qualified expenses (tuition, fees, books, computers) then some coordination of expenses* will have to be done with the student and/or the other parent, but only if the other parent is claiming the student as a dependent.  If that is the case, we need more info: 

• Who is claiming the student as a dependent
• Box 1 of the 1098-T
• box 5 of the 1098-T
• Any other scholarships not shown in box 5
• Does box 5 include any of the 529/ESA plan payments (it should not)
• Is any of the Scholarship restricted; i.e. it must be used for tuition
• Box 1 of the 1099-Q
• Box 2 of the 1098-Q
• Who’s name and SS# are on the 1099-Q, parent or student (who’s the “recipient”)?
• Room & board paid. If student lives off campus, what is school's R&B charge. If the student lives at home, only the school's board charge for on campus students. 
• Other qualified expenses not included in box 1 of the 1098-T, e.g. books & computers
• How much taxable income does the student have, from what sources
• Are you, the other parent, or the student trying to claim the tuition credit (are you eligible)?
• Is the student an undergrad or grad student?

*There are three things you can do with your Qualified educational expenses (QEE):

1. Allocate then to scholarships (so that the scholarship remains tax free)
2. Use them to claim an education credit
3. Allocate them to the 529 distribution (1099-Q) so that it will not all be taxable